Optical Fiber Cable

A TOSLINK optical fiber cable with a clear jacket. These cables are used mainly for digital audio connections between devices.

An optical fiber cable, also known as fiber optic cable, is an assembly similar to an electrical cable, but containing one or more optical fibers that are used to carry light. The optical fiber elements are typically individually coated with plastic layers and contained in a protective tube suitable for the environment where the cable will be deployed. Different types of cable are used for different applications, for example long distance telecommunication, or providing a high-speed data connection between different parts of a building.

Design

A Multi-Fiber Cable

Optical fiber consists of a core and a cladding layer, selected for total internal reflection due to the difference in the refractive index between the two. In practical fibers, the cladding is usually coated with a layer of acrylate polymer or polyimide. This coating protects the fiber from damage but does not contribute to its optical waveguide properties. Individual coated fibers (or fibers formed into ribbons or bundles) then have a tough resin buffer layer and/or core tube(s) extruded around them to form the cable core. Several layers of protective sheathing, depending on the application, are added to form the cable. Rigid fiber assemblies sometimes put light-absorbing (“dark”) glass between the fibers, to prevent light that leaks out of one fiber from entering another. This reduces cross-talk between the fibers, or reduces flare in fiber bundle imaging applications.

For indoor applications, the jacketed fiber is generally enclosed, with a bundle of flexible fibrous polymer strength members like aramid (e.g. Twaron or Kevlar), in a lightweight plastic cover to form a simple cable. Each end of the cable may be terminated with a specialized optical fiber connector to allow it to be easily connected and disconnected from transmitting and receiving equipment.

Fiber-optic cable in a Telstra pit

Investigating a fault in a fiber cable junction box. The individual fiber cable strands within the junction box are visible.

An optical fiber breakout cable

For use in more strenuous environments, a much more robust cable construction is required. In loose-tube construction the fiber is laid helically into semi-rigid tubes, allowing the cable to stretch without stretching the fiber itself. This protects the fiber from tension during laying and due to temperature changes. Loose-tube fiber may be “dry block” or gel-filled. Dry block offers less protection to the fibers than gel-filled, but costs considerably less. Instead of a loose tube, the fiber may be embedded in a heavy polymer jacket, commonly called “tight buffer” construction. Tight buffer cables are offered for a variety of applications, but the two most common are “Breakout” and “Distribution”. Breakout cables normally contain a ripcord, two non-conductive dielectric strengthening members (normally a glass rod epoxy), an aramid yarn, and 3 mm buffer tubing with an additional layer of Kevlar surrounding each fiber. The ripcord is a parallel cord of strong yarn that is situated under the jacket(s) of the cable for jacket removal. Distribution cables have an overall Kevlar wrapping, a ripcord, and a 900 micrometer buffer coating surrounding each fiber. These fiber units are commonly bundled with additional steel strength members, again with a helical twist to allow for stretching.

A critical concern in outdoor cabling is to protect the fiber from contamination by water. This is accomplished by use of solid barriers such as copper tubes, and water-repellent jelly or water-absorbing powder surrounding the fiber.

Finally, the cable may be armored to protect it from environmental hazards, such as construction work or gnawing animals. Undersea cables are more heavily armored in their near-shore portions to protect them from boat anchors, fishing gear, and even sharks, which may be attracted to the electrical power that is carried to power amplifiers or repeaters in the cable.

Modern cables come in a wide variety of sheathings and armor, designed for applications such as direct burial in trenches, dual use as power lines, installation in conduit, lashing to aerial telephone poles, submarine installation, and insertion in paved streets.

Capacity and Market

In September 2012, NTT Japan demonstrated a single fiber cable that was able to transfer 1 petabit per second (1015bits/s) over a distance of 50 kilometers.

Modern fiber cables can contain up to a thousand fibers in a single cable, with potential bandwidth in the terabytes per second. In some cases, only a small fraction of the fibers in a cable may be actually “lit”. Companies can lease or sell the unused fiber to other providers who are looking for service in or through an area. Companies may “overbuild” their networks for the specific purpose of having a large network of dark fiber for sale, reducing the overall need for trenching and municipal permitting. They may also deliberately under-invest to prevent their rivals from profiting from their investment.

Reliability and Quality

Optical fibers are very strong, but the strength is drastically reduced by unavoidable microscopic surface flaws inherent in the manufacturing process. The initial fiber strength, as well as its change with time, must be considered relative to the stress imposed on the fiber during handling, cabling, and installation for a given set of environmental conditions. There are three basic scenarios that can lead to strength degradation and failure by inducing flaw growth: dynamic fatigue, static fatigues, and zero-stress aging.

Telcordia GR-20, Generic Requirements for Optical Fiber and Optical Fiber Cable, contains reliability and quality criteria to protect optical fiber in all operating conditions. The criteria concentrate on conditions in an outside plant (OSP) environment. For the indoor plant, similar criteria are in Telcordia GR-409, Generic Requirements for Indoor Fiber Optic Cable.

Cable Types

This section needs expansion. You can help by adding to it. (June 2008)

OFC: Optical fiber, conductive

OFN: Optical fiber, nonconductive

OFCG: Optical fiber, conductive, general use

OFNG: Optical fiber, nonconductive, general use

OFCP: Optical fiber, conductive, plenum

OFNP: Optical fiber, nonconductive, plenum

OFCR: Optical fiber, conductive, riser

OFNR: Optical fiber, nonconductive, riser

OPGW: Optical fiber composite overhead ground wire

ADSS: All-Dielectric Self-Supporting

OSP: Fiber optic cable, outside plant

MDU: Fiber optics cable, multiple dwelling unit

Jacket Material

The jacket material is application specific. The material determines the mechanical robustness, aging due to UV radiation, oil resistance, etc. Nowadays PVC is being replaced by halogen free alternatives, mainly driven by more stringent regulations.

Fiber Material

There are two main types of material used for optical fibers: glass and plastic. They offer widely different characteristics and find uses in very different applications. Generally, plastic fiber is used for very short range and consumer applications, glass fiber is used for short/medium range (multi-mode) and long range (single-mode) telecommunications.

Color Coding

Patch Cords

The buffer or jacket on patchcords is often color-coded to indicate the type of fiber used. The strain relief “boot” that protects the fiber from bending at a connector is color-coded to indicate the type of connection. Connectors with a plastic shell (such as SC connectors) typically use a color-coded shell. Standard color codings for jackets (or buffers) and boots (or connector shells) are shown below:

Cord Jacket (or Buffer) Color

Connector Boot (or Shell) Colors

Remark: It is also possible that a small part of a connector is additionally color-coded, e.g. the lever of an E-2000 connector or a frame of an adapter. This additional colour coding indicates the correct port for a patchcord, if many patchcords are installed at one point.

Multi-fiber Cables

Individual fibers in a multi-fiber cable are often distinguished from one another by color-coded jackets or buffers on each fiber. The identification scheme used by Corning Cable Systems is based on EIA/TIA-598, “Optical Fiber Cable Color Coding.” EIA/TIA-598 defines identification schemes for fibers, buffered fibers, fiber units, and groups of fiber units within outside plant and premises optical fiber cables. This standard allows for fiber units to be identified by means of a printed legend. This method can be used for identification of fiber ribbons and fiber subunits. The legend will contain a corresponding printed numerical position number and/or color for use in identification.

In the UK the colour codes for COF200 and 201 are different. Each 12 fibre bundle or element within a Cable Optical Fibre 200/201 cable is coloured as follows:

Blue

Orange

Green

Red

Grey

Yellow

Brown

Violet

Black

White

Pink

Turquoise

Each element is in a tube within the cable (not a blown fibre tube) The cable elements start with the red tube and are counted around the cable to the green tube. Active elements are in white tubes and yellow fillers or dummies are laid in the cable to fill it out depending on how many fibres and units exists – can be up to 276 fibres or 23 elements for external cable and 144 fibres or 12 elements for internal. The cable has a central strength member normally made from fiberglass or plastic. There is also a copper conductor in external cables.

Propagation Speed and Delay

Optical cables transfer data at the speed of light in glass. This is the speed of light in vacuum divided by the refractive index of the glass used, typically around 180,000 to 200,000 km/s, resulting in 5.0 to 5.5 microseconds of latency per km. Thus the round-trip delay time for 1000 km is around 11 milliseconds.

Losses

Typical modern multimode graded-index fibers have 3 dB/km of attenuation loss (50% loss per km) at 850 nm and 1 dB/km at 1300 nm. Singlemode 9/125 loses 0.4 dB/km at 1310 nm and 0.25 dB/km at 1550 nm. Very high quality singlemode fiber intended for long distance applications is specified at a loss of 0.19 dB/km at 1550 nm. POF (plastic optical fiber) loses much more: 1 dB/m at 650 nm. Plastic optical fiber is large core (about 1mm) fiber suitable only for short, low speed networks such as within cars.

Each connection made adds about 0.6 dB of average loss, and each joint (splice) adds about 0.1 dB. Depending on the transmitter power and the sensitivity of the receiver, if the total loss is too large the link will not function reliably.

Invisible IR light is used in commercial glass fiber communications because it has lower attenuation in such materials than visible light. However, the glass fibers will transmit visible light somewhat, which is convenient for simple testing of the fibers without requiring expensive equipment. Splices can be inspected visually, and adjusted for minimal light leakage at the joint, which maximizes light transmission between the ends of the fibers being joined.

The charts at “Understanding wavelengths In fiber optics” and “Optical power loss (attenuation) in fiber” illustrate the relationship of visible light to the IR frequencies used, and show the absorption water bands between 850, 1300 and 1550 nm.

Safety

The infrared light used in telecommunications cannot be seen, so there is a potential laser safety hazard to technicians. The eye’s natural defense against sudden exposure to bright light is the blink reflex, which is not triggered by infrared sources. In some cases the power levels are high enough to damage eyes, particularly when lenses or microscopes are used to inspect fibers that are emitting invisible infrared light. Inspection microscopes with optical safety filters are available to guard against this. More recently indirect viewing aids are used, which can comprise a camera mounted within a handheld device, which has an opening for the connectorized fiber and a USB output for connection to a display device such as a laptop. This makes the activity of looking for damage or dirt on the connector face much safer.

Small glass fragments can also be a problem if they get under someone’s skin, so care is needed to ensure that fragments produced when cleaving fiber are properly collected and disposed of appropriately.

Hybrid Cables

There are hybrid optical and electrical cables that are used in wireless outdoor Fiber To The Antenna (FTTA) applications. In these cables, the optical fibers carry information, and the electrical conductors are used to transmit power. These cables can be placed in several environments to serve antennas mounted on poles, towers, and other structures.

According to Telcordia GR-3173, Generic Requirements for Hybrid Optical and Electrical Cables for Use in Wireless Outdoor Fiber To The Antenna (FTTA) Applications, these hybrid cables have optical fibers, twisted pair/quad elements, coaxial cables and/or current-carrying electrical conductors under a common outer jacket. The power conductors used in these hybrid cables are for directly powering an antenna or for powering tower-mounted electronics exclusively serving an antenna. They have a nominal voltage normally less than 60 VDC or 108/120 VAC. Other voltages may be present depending on the application and the relevant National Electrical Code (NEC).

These types of hybrid cables may also be useful in other environments such as Distributed Antenna System (DAS) plants where they will serve antennas in indoor, outdoor, and roof-top locations. Considerations such as fire resistance, Nationally Recognized Testing Laboratory (NRTL) Listings, placement in vertical shafts, and other performance-related issues need to be fully addressed for these environments.

Since the voltage levels and power levels used within these hybrid cables vary, electrical safety codes consider the hybrid cable to be a power cable, which needs to comply with rules on clearance, separation, etc.

Innerducts

Innerducts are installed in existing underground conduit systems to provide clean, continuous, low-friction paths for placing optical cables that have relatively low pulling tension limits. They provide a means for subdividing conventional conduit that was originally designed for single, large-diameter metallic conductor cables into multiple channels for smaller optical cables.

Types

Innerducts are typically small-diameter, semi-flexible subducts. According to Telcordia GR-356, there are three basic types of innerduct: smoothwall, corrugated, and ribbed. These various designs are based on the profile of the inside and outside diameters of the innerduct. The need for a specific characteristic or combination of characteristics, such as pulling strength, flexibility, or the lowest coefficient of friction, dictates the type of innerduct required.

Beyond the basic profiles or contours (smoothwall, corrugated, or ribbed), innerduct is also available in an increasing variety of multiduct designs. Multiduct may be either a composite unit consisting of up to four or six individual innerducts that are held together by some mechanical means, or a single extruded product having multiple channels through which to pull several cables. In either case, the multiduct is coilable, and can be pulled into existing conduit in a manner similar to that of conventional innerduct.

Placement

Innerducts are primarily installed in underground conduit systems that provide connecting paths between manhole locations. In addition to placement in conduit, innerduct can be directly buried, or aerially installed by lashing the innerduct to a steel suspension strand.

As stated in GR-356, cable is typically placed into innerduct in one of three ways. It may be

Pre-installed by the innerduct manufacturer during the extrusion process,

Pulled into the innerduct using a mechanically assisted pull line, or

Blown into the innerduct using a high air volume cable blowing apparatus.

Fiber Optic Coupler

A fiber optic coupler is a device used in optical fiber systems with one or more input fibers and one or several output fibers. Light entering an input fiber can appear at one or more outputs and its power distribution potentially depending on the wavelength and polarization. Such couplers can be fabricated in different ways, for example by thermally fusing fibers so that their cores get into intimate contact. If all involved fibers are single-mode (supporting only a single mode per polarization direction for a given wavelength), there are certain physical restrictions on the performance of the coupler. In particular, it is not possible to combine two or more inputs of the same optical frequency into one single-polarization output without significant excess losses. However, such a restriction does not occur for different input wavelengths: there are couplers that can combine two inputs at different wavelengths into one output without exhibiting significant losses. Wavelength-sensitive couplers are used as multiplexers in wavelength-division multiplexing (WDM) telecom systems to combine several input channels with different wavelengths, or to separate channels.

Optical Power Meter

Optical power meter in use

An optical power meter (OPM) is a device used measure the power in an optical signal. The term usually refers to a device for testing average power in fiber optic systems. Other general purpose light power measuring devices are usually called radiometers, photometers, laser power meters (can be photodiode sensors or thermopile laser sensors), light meters or lux meters.

A typical optical power meter consists of a calibrated sensor, measuring amplifier and display. The sensor primarily consists of a photodiode selected for the appropriate range of wavelengths and power levels. On the display unit, the measured optical power and set wavelength is displayed. Power meters are calibrated using a traceable calibration standard such as a NIST standard.

A traditional optical power meter responds to a broad spectrum of light, however the calibration is wavelength dependent. This is not normally an issue, since the test wavelength is usually known, however it has a couple of drawbacks. Firstly, the user must set the meter to the correct test wavelength, and secondly if there are other spurious wavelengths present, then wrong readings will result.

Sometimes optical power meters are combined with a different test function such as an Optical Light Source (OLS) or Visual Fault Locator (VFL), or may be a sub-system in a much larger instrument. When combined with a light source, the instrument is usually called an Optical Loss Test Set.

Optical Loss Test Sets (OLTS) are available in dedicated hand held instruments and platform-based modules to suit various network architectures and test requirements. They are used to measure optical power and power loss, and reflectance and reflected power loss. The products may also be used as optical sources or optical power meters, or to measure optical return loss or event reflectance.

Three types of equipment can be used to measure optical power loss:

Component equipment – Optical Power Meters (OPMs) and Stabilized Light Sources (SLSs) are packaged separately, but when used together they can provide a measurement of end-to-end optical attenuation over an optical path. Such component equipment can also be used for other measurements.

Integrated test set – When an SLS and OPM are packaged in one unit, it is called an integrated test set. Traditionally, an integrated test set is usually called an OLTS. GR-198, Generic Requirements for Hand-Held Stabilized Light Sources, Optical Power Meters, Reflectance Meters, and Optical Loss Test Sets, discusses OLTS equipment in depth.

An Optical Time Domain Reflectometer (OTDR) can be used to measure optical link loss if its markers are set at the terminus points for which the fiber loss is desired. However a single-direction measurement may not be accurate if there are multiple fibers in a link, since the back-scatter coefficient is variable between fibers. The accuracy of such a measurement can be increased if the measurement is made as a bidirectional average of the fiber. GR-196, Generic Requirements for Optical Time Domain Reflectometer (OTDR) Type Equipment, discusses OTDR equipment in depth.

Optical Time-Domain Reflectometer (OTDR)

An OTDR

An OTDR in use

An Optical Time-Domain Reflectometer (OTDR) is an optoelectronic instrument used to characterize an optical fiber. An OTDR is the optical equivalent of an electronic time domain reflectometer. It injects a series of optical pulses into the fiber under test and extracts, from the same end of the fiber, light that is scattered (Rayleigh backscatter) or reflected back from points along the fiber. The scattered or reflected light that is gathered back is used to characterize the optical fiber. This is equivalent to the way that an electronic time-domain meter measures reflections caused by changes in the impedance of the cable under test. The strength of the return pulses is measured and integrated as a function of time, and plotted as a function of fiber length.

Parallel Optical Interface

A Parallel Optical Interface is a form of fiber optic technology aimed primarily at communications and networking over relatively short distances (less than 300 meters), and at high bandwidths.

Parallel optic interfaces differ from traditional fiber optic communication in that data is simultaneously transmitted and received over multiple fibers. Different methods exist for splitting the data over this high bandwidth link. In the simplest form, the parallel optic link is a replacement for many serial data communication links. In the more typical application, one byte of information is split up into bits and each bit is coded and sent across the individual fibers. Needless to say, there are many ways to perform this multiplexing provided the fundamental coding at the fiber level meets the channel requirement.

The main applications for parallel optical interfaces are found in telecommunications and supercomputers, also being introduced to consumer applications. It displaces copper backplanes that are commonly used for large switching equipment design.

There are two forms of commercially available products for parallel optic interfaces. The first is a twelve channel system consisting of an optical transmitter and an optical receiver. The second is a four channel transceiver product that is capable of transmitting four channels and receiving four channels in one product.

Parallel optics is often the most cost effective solution for getting 40 Gigabit per second transmission of data over distances exceeding 100 meters. 100GE Optical Transceiver comes with 100 Gigabit of data transmit. Data is delivered in both duplex and parallel mechanism with 100GE.